1. Field of the Invention
The present invention relates to an image processing method and to an image processing apparatus for stabilizing output characteristics of an output apparatus, and to a recording medium therefor.
2. Description of the Related Art
In recent years, various types of peripheral apparatuses, such as personal computers and printers, have become popular, and anyone can easily produce hardcopy output of word-processor documents and graphic images produced by a computer. As a representative construction thereof, an image processing system such as that shown in FIG. 10 is known.
FIG. 10 shows an outline of a construction of a system which produces a page layout document such as DTP, and word-processor and graphic documents by using a host computer 101 and which produces hardcopy output by a laser printer, an ink jet printer, etc.
Reference number 102 denotes an application which operates in a host computer. Representative well known examples are word processor software such as MS Word (trademark) of Microsoft Corporation, and page layout software such as PageMaker (trademark) of Adobe Systems Incorporated.
A digital document produced by this software is passed to a printer driver 103 via an operating system (OS) of a computer (not shown).
The digital document is usually expressed as a set of command data representing figures and characters which constitute one page, and these commands are sent to the printer driver 103. The series of commands forming the screen are expressed as a language system called xe2x80x9cPDL (Page Description Language)xe2x80x9d. Representative well known examples of PDL are GDI (Graphics Device Interface) (trademark) and PS (PostScript) (trademark).
The printer driver 103 transfers the received PDL command to a rasterizer 105 inside a raster image processor 104. The rasterizer 105 forms the characters, figures, etc., expressed by PDL commands, into a two-dimensional bit-mapped image, performs a gradation correction process on each pixel by using a look-up table, and performs a quantization process, such as a dithering process. Since the bit-mapped image is an image in which a two-dimensional plane is filled with a repetition of a one-dimensional raster (line), the rasterizer 105 is called a xe2x80x9crasterizerxe2x80x9d. The expanded bit-mapped image is temporarily stored in an image memory 106.
The above operation is schematically shown in FIG. 11. A document image 111 displayed on the host computer is sent, as a PDL command sequence 112, to the rasterizer via the printer driver. The rasterizer expands the two-dimensional bit-mapped image onto the image memory, as indicated by reference numeral 113.
The expanded image data is sent to a color printer 107. In the color printer 107, an image forming unit 108 employing a well-known electrophotographic method or ink-jet recording method is used, and by using this, a visible image is formed on paper and a printout is produced. It is a matter of course that the image data in the image memory is transferred in synchronization with a synchronization signal (not shown), a clock signal (not shown), or a transfer request (not shown) of a specific color-component signal, which is required to operate the image forming unit.
In the conventional technology such as that described above, when an image forming unit used for output is considered, it is clear that various problems will occur.
The problems arise from instability of image output characteristics of the image forming unit and from variations among apparatuses, and in output images of the same original document, hue changes each time an output is made or differs when the output is made by a different printer.
This occurs due to the following reasons. For example, when an electrophotographic method is used in the image forming unit, steps, such as laser exposure, latent-image formation on a photosensitive body, toner development, toner transfer onto a paper medium, and fixing by heat, in the electrophotographic process are affected by environmental temperature and humidity or by factors such as aging of components, and the amount of toner which is finally fixed onto the paper changes for each situation.
Such instability is not characteristic of the electrophotographic method, and it is known that such instability occurs in a similar manner even in an ink-jet recording method, a heat-sensitive transfer method, and various other methods.
In order to overcome such problems, conventionally, a system shown in FIG. 12 is conceived. This is designed to output a test pattern image, such as that indicated by reference numeral 121, from the printer 107, to measure the density of the output pattern, and to correct the characteristics of the image forming unit. In this system, a look-up table which is used in a gradation correction process performed by the rasterizer 105 is created.
The operations at this time are described below step by step.
Initially, the host computer 101 sends a command for outputting a predetermined gradation pattern to the raster image processor 104. The raster image processor 104 generates a bit-mapped pattern for printer output on the basis of the sent command and transfers it to the printer section 107. The printer section 107 outputs the supplied bit-mapped pattern on a paper medium. Here, for the output pattern, it is assumed that a pattern is output such that a toner adherence area ratio changes in eight steps from 0% to 100% with respect to cyan (C), magenta (M), yellow (Y), and black (K) corresponding to a four-color toner of a printer as indicated by the test pattern image 121. In FIG. 12, each of the eight steps is given a number from 0 to 7, and the gradation pattern of each color is such that a horizontal row of reference numeral 122 is for C, a horizontal row of reference numeral 123 is for M, a horizontal row of reference numeral 124 is for Y, and a horizontal row of reference numeral 125 is for K.
In the output pattern, there are a total of 32 rectangular printing areas (patch areas) in 4 colorsxc3x978 steps, and each of the areas is measured by using a reflection densitometer 126. The measured value (the density value of each patch) is sent to the host computer.
The host computer compares the measured value with a prestored reference value, creates a correction table for each of the colors C, M, Y, and K, and registers this table in a table conversion section of the raster image processor. The table conversion section corrects a value which is written as bit-mapped data when the raster image processor creates a bit-mapped image.
For example, in a case where the density of the third patch of cyan in the test pattern 121 is measured lower than the reference value, in the correction table, by correcting the bit-mapped data corresponding to the third patch of cyan to a high value, the density characteristics of the printer can be brought closer to the reference value.
This state is shown in FIGS. 13A and 13B. FIG. 13A shows that the density value obtained for a cyan patch is plotted with respect to the gradation number of the patch. FIG. 13B shows a correction table which is created based on this measured value.
In FIG. 13A, the horizontal axis indicates a gradation number, the vertical axis indicates a measured value, ∘ marks 131 each indicate a measured value, and a curve 132 connects the measured values by straight lines.
The gradation number along the horizontal axis is considered here. This is equivalent to that signal values to be output to the printer which is an image forming unit are sampled at predetermined intervals and are assigned numbers. A conventional printer unit is capable of image output at the number of gradations formed of eight bits for each of C, M, Y, and K, and forms and outputs an image having continuous gradations on paper by using a binarization process using a well-known dithering process according to the level of each signal value.
In the embodiment described here, since patches of 0 to 7 are output by signal values in which an 8-bit signal of 0 to 255 are divided evenly, the horizontal axis of FIG. 13 directly shows signal values for forming an image by the printer, and the following correspondence is satisfied:
Gradation No. 0=Printer output signal value 0
Gradation No. 1=Printer output signal value 36
Gradation No. 2=Printer output signal value 73
Gradation No. 3=Printer output signal value 109
Gradation No. 4=Printer output signal value 146
Gradation No. 5=Printer output signal value 182
Gradation No. 6=Printer output signal value 219
Gradation No. 7=Printer output signal value 255
A thick line 133 in FIG. 13 shows an example of ideal density characteristics which should be taken by the density value of the output patch with respect to the signal value which is output by the printer. That is, it is preferable that the printer have density characteristics which are proportional to the output signal value and which reach a predetermined maximum density value (1.6 in the figure) when the output signal value is at a maximum (255). However, the printer has density characteristics such as those indicated by a curve 132 in FIG. 13A due to variations in individual printers and due to environmental variations.
Accordingly, when a bit-mapped image of the printer output signal values of C, M, Y, and K is created by rasterizing PDL commands, the raster image processor may obtain bit-mapped data by correcting the C, M, Y, and K values by using a predetermined look-up table.
This look-up table may be a table having characteristics inverse to those of the curve 132 in FIG. 13A. The host computer computes a conversion table having such characteristics for each of C, M, Y, and K on the basis of the measured density value and transfers it to the raster image processor.
A curve 134 in FIG. 13B shows that an actual conversion table is plotted, and has characteristics inverse to those of the curve 132 in FIG. 13A, that is, characteristics such that the curve 132 is folded symmetrically with respect to a straight line 133. The curve 134 is formed of an ordinary look-up table, and the rasterizer uses a signal value (here, a signal C) after rasterization as an input signal for the look-up table and uses the output value of the look-up table as a signal value (here, a signal Cxe2x80x2) to be written into the bit-mapped data.
In a case where the image forming unit can produce output only by binary values of xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d, a well-known pseudo-halftone process, such as a dithering process, is further performed on the signal Cxe2x80x2, after which the signal is written into the bit-map memory.
Here, the characteristics of the curve 134 show that the output signal value when the input signal has a value 255, is smaller than 255, as indicated by a point 135. The reason for this is that, at the curve 132 in FIG. 13A, the values of Dc6 and Dc7 are larger than the expected maximum density value.
Therefore, when an output apparatus having such density characteristics is used, the signal Cxe2x80x2 after gradation correction does not become 255. Here, when the output signal value to the printer is 0, a toner or ink image is not formed and the output image becomes white. When the output signal value is 255, a solidly filled image is formed by toner or ink, and the output image becomes black (or solidly filled areas of cyan, magenta, etc.). However, since the signal value after gradation correction does not become 255, as a result of this value being subjected to a dithering process, the toner or ink formation signal after the dithering process is converted into a pattern in which 0 and 1 occur alternately, making it impossible to form a solidly filled image on a printout.
For example, when a solidly filled thin line is reproduced, the thin line in the output image may be broken.
The present invention has been achieved in view of the above-described points. An object of the present invention is to satisfactorily reproduce an image which should be output as a solidly filled image.
Another object of the present invention is to easily create correction conditions having different reproducibilities.
To achieve the above-mentioned objects, according to one aspect of the present invention, there is provided an image processing method comprising the steps of inputting data obtained by reading a patch pattern formed by an image forming apparatus; and creating correction conditions for the image forming apparatus on the basis of the data, wherein the correction conditions are created based on the data so that output image data with respect to a specific gradation level of multi-level gradation input image data satisfy preset conditions.
According to another aspect of the present invention, there is provided an image processing method comprising the steps of inputting data obtained by reading a patch pattern formed by an image forming apparatus; and creating correction conditions for the image forming apparatus on the basis of the data, wherein a plurality of methods of creating the correction conditions are provided, and the respective methods realize different reproducibilities.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.